High temperature dynamic yielding in metastable Ti-6.8Mo-4.5Fe-1.5Al

The discontinuous yield behavior of TIMETAL LCB (Ti-6.8Mo-4.5Fe-1.5AI), a prototypic metastable beta titanium alloy, has been examined between 840 and 1290 degrees C over strain rates of 2 x 10(-4)-1 s(-1). These studies have shown that the magnitude of the discontinuous yield drop exhibits, at constant strain rate, a broad maximum with increasing test temperature, the temperature of this maximum increasing with increasing strain rate. Incremental straining experiments indicate that the discontinuous yield phenomena observed is associated with mobile dislocation generation from grain boundaries, plastic deformation then spreading inwards towards the grain interior with increasing strain. These observations are consistent with the dynamic approach to Johnston and Gilman's dynamic yield theory-that grain boundaries act as sources for mobile dislocation generation. Activation analysis further suggests that the temperature dependence of the discontinuous yield drop is related to unlocking of dislocation sources through localized dislocation climb, boundary migration and/or solute rearrangement. (C) 1998 Elsevier Science S.A. All rights reserved.